a) Field of the Invention
The invention is directed to a device for generating flows of gas for filtering the radiation emitted in plasma-based radiation sources in which at least one supersonic slit nozzle is provided for generating a gas curtain in the beam bundle that can be coupled out.
b) Description of the Related Art
The invention is preferably applied in semiconductor chip fabrication in radiation sources for EUV lithography for protecting the collector optics and other optics downstream of the latter from debris.
As per the current state of the art, the structuring of semiconductor chips is carried out by means of optical lithography. To this end, the desired structure contained on a mask is imaged onto a semiconductor wafer. A light-sensitive coating on this semiconductor wafer undergoes chemical changes through exposure and accordingly allows further selective working of exposed and unexposed surfaces. The spatial resolution that can be achieved by this method is limited by the wavelength of the light that is used. Progressive miniaturization requires a continual increase in resolution which can only be achieved through a reduction in the light wavelength. Extreme ultraviolet (EUV) radiation with a wavelength of 13.5 nm is provided as a substitute for the formerly used DUV excimer lasers with a wavelength of 193 nm. Plasma-based radiation sources must be resorted to for generating these wavelengths. Because every known material is highly absorbent for EUV radiation, the entire process from generation of radiation to exposure must be carried out under high vacuum using reflecting optics.
High-power radiation sources for EUV radiation are based on a luminous plasma that is generated by a laser pulse (LPP—Laser-Produced Plasma) or a gas discharge (GDP—Gas Discharge Plasma). In so doing, a material, e.g., xenon, tin or lithium, is heated to temperatures of several hundred thousand degrees so that the occurring plasma is emitted in the EUV region. This radiation is collected by means of collector optics and imaged in an intermediate focus which constitutes the interface to the adjoining exposure module. Aside from achieving the highest possible radiation output in the wavelength region around 13.5 nm, the life of the plasma-generating components of the radiation source and of the collector optics is of primary importance. The plasma emits not only the desired radiation but also high-energy particles and larger clusters, collectively referred to as debris. Depending on the operating parameters, the emitted debris leads to abrasion of reflecting layers, deposits of impurities, or a coarsening of the optical surface. All of these processes reduce the reflectivity of the optics. Therefore, devices for protecting the collector optics from debris play a key role in the successful use of high-power EUV sources.
The following basic principles are known for protecting the collector optics:    a) mechanically moving apertures and shutters that pass the radiation pulse and seal against slower debris particles    b) stationary or rotating arrangements of fins or plates disclosed, e.g., in EP 1 274 287 A1 and EP 1 391 785 A1 having fins or plates which are oriented substantially exclusively in radial direction toward the locus of emission and which capture the debris particles by adhesion    c) electric and/or magnetic fields (usually in combination with filters for uncharged particles) such as are described, e.g., in U.S. Pat. No. 6,881,971 B2    d) spaces which are filled with buffer gas and in which debris particles are decelerated by collision with gas particles (and are eliminated through suction or other filter means as described, e.g., in DE 10 2005 020 521 A1), and    e) buffer gas curtains forming a fast, flat lateral gas flow which decelerate and deflect debris particles (see WO 2003/26363 A1).
Basically, these same debris filter concepts are used for GDP sources and LPP sources, but with the difference that in GDP sources the plasma occurs in the immediate spatial vicinity of an electrode system so that only a limited solid angle of emitted radiation is available, whereas in LPP sources substantially larger radiation angles must be covered.
It has long been known to employ streaming buffer gases in the evacuated chambers for generating plasma in combination with mechanical and field-coupled debris filters because the gas particles decelerate and/or deflect the debris particles by colliding with them and appreciably improve the action of additional debris filters. However, an efficient evacuation system must be used to maintain a reduced extinction effect of the buffer gas for the emitted EUV radiation.
The use of a gas curtain, as is mentioned above (e) and described in WO 2003/26363 A1, is a particularly efficient way to filter debris. When implemented with supersonic nozzles, a gas curtain of this kind can even deflect larger debris clusters adequately and hardly interferes with the vacuum needed to generate plasma. Further, it requires little space and can be used in the immediate vicinity of the plasma.
However, the problem with using a gas curtain is that its extension (within a plane) is spatially limited so that the generating slit nozzle must be moved as close as possible to the plasma in order to cover a large solid angle of the EUV radiation. This results in a high thermal stress on the nozzle. However, supersonic slit nozzles comprise extremely precise, highly engineered shapes which are difficult to cool and extremely difficult to produce from high-melting materials (e.g., tungsten or molybdenum).